This invention relates in general to hybrid drive systems, such as are used in conjunction with drive train systems for vehicles. In particular, this invention relates to an improved control apparatus and method for operating a combined hybrid drive and brake system in such a manner that transitions between various operating modes occur in a smooth and unnoticeable manner.
Drive train systems are widely used for generating power from a source and for transferring such power from the source to a driven mechanism. Frequently, the source generates rotational power, and such rotational power is transferred from the source of rotational power to a rotatably driven mechanism. For example, in most land vehicles in use today, an engine generates rotational power, and such rotational power is transferred from an output shaft of the engine through a driveshaft to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle.
In some of these land vehicles and other mechanisms, a hybrid drive system (also known as an energy recovery system) is provided in conjunction with the drive train system to decelerate the rotatably driven mechanism, accumulate the energy resulting from such deceleration, and use the accumulated energy to subsequently accelerate the rotatably driven mechanism. To accomplish this, a typical hybrid drive system includes a reversible energy transfer machine that is coupled to the drive train system and an energy storage device that communicates with the reversible energy transfer machine. To decelerate the vehicle, the hybrid drive system is operated in a retarding mode, wherein the reversible energy transfer machine slows the rotation of the rotatably driven mechanism and stores the kinetic energy of the vehicle in the energy storage device as potential energy. To subsequently accelerate the vehicle, the hybrid drive system is operated in a driving mode, wherein the potential energy stored in the energy storage device is supplied to the reversible energy transfer machine to rotatably drive the rotatably driven mechanism.
Although hybrid drive systems of this general type function in an energy-efficient manner, it is often necessary or desirable to provide a separate brake system to affirmatively slow or stop the rotation of the rotatably driven mechanism in certain situations. For example, when used in conjunction with the drive train system of a vehicle that is relatively heavy or moving relatively fast, the hybrid drive system may not always have the capacity to adequately retard the rotation of the rotatably driven mechanism as quickly as requested by a driver. Additionally, when used in conjunction with the drive train system of a vehicle that is stopped on an inclined surface, the hybrid drive system cannot positively stop the rotatably driven mechanism to prevent any movement of the vehicle. To address these and other situations, the separate brake system (which can be embodied as a conventional pneumatically or hydraulically actuated friction brake system) is often provided in conjunction with the hybrid drive system. In such a combined hybrid drive and brake system, the hybrid drive system can be actuated to normally retard the rotation of the rotatably driven mechanism in the energy-efficient manner described above, and the brake system can be actuated when otherwise necessary.
In a combined hybrid drive and brake system such as described above, deceleration of the rotatably driven mechanism can be accomplished by either (1) the hybrid drive system operating alone, (2) the brake system operating alone, or (3) both the hybrid drive system and the brake system operating in combination. The selection of which of these three operating modes is appropriate can be determined by a control apparatus in accordance with a variety of parameters. Because these parameters can (and typically do) change during the deceleration of the rotatably driven mechanism, the control apparatus will frequently transition between two or more of the three operating modes of the combined hybrid drive and brake system. Thus, it would be desirable to provide an improved control apparatus and method for operating a combined hybrid drive and brake system in such a manner that the transitions between these various operating modes occur in a smooth and unnoticeable manner.